Abstract
Influential demographic projections suggest that the global human population will stabilize at about 9–10 billion people by mid-century. These projections rest on two fundamental assumptions. The first is that the energy needed to fuel development and the associated decline in fertility will keep pace with energy demand far into the future. The second is that the demographic transition is irreversible such that once countries start down the path to lower fertility they cannot reverse to higher fertility. Both of these assumptions are problematic and may have an effect on population projections. Here we examine these assumptions explicitly. Specifically, given the theoretical and empirical relation between energy-use and population growth rates, we ask how the availability of energy is likely to affect population growth through 2050. Using a cross-country data set, we show that human population growth rates are negatively related to per-capita energy consumption, with zero growth occurring at ∼13 kW, suggesting that the global human population will stop growing only if individuals have access to this amount of power. Further, we find that current projected future energy supply rates are far below the supply needed to fuel a global demographic transition to zero growth, suggesting that the predicted leveling-off of the global population by mid-century is unlikely to occur, in the absence of a transition to an alternative energy source. Direct consideration of the energetic constraints underlying the demographic transition results in a qualitatively different population projection than produced when the energetic constraints are ignored. We suggest that energetic constraints be incorporated into future population projections.
Highlights
Over human evolutionary history, the global human population has grown from an initial small size to,7 billion today
All attempts to project the future size of the global human population are subject to considerable, and unavoidable, uncertainty [23]
The bulk of this uncertainty pertains to the rate and timing of the demographic transition
Summary
The global human population has grown from an initial small size to ,7 billion today. Demographic studies suggest that we can expect a leveling-off of the human population at about 9–10 billion by the middle of this century [2,3] Such projections are made by extrapolating recent trends in the relationship between time and vital rates. Growth rates decline as a result of economic development, which brings benefits that increase lifespan and reduce infant mortality [6,7]. Stimulated by these development benefits, fertility rates decline until they reach replacement levels or lower
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